Method of write-through operation of direct viewing bistable storage tube to produce nonstored image of high brightness during shortage of another image



F 25. 1969 c. N. WINNINGSTAD 3,

METHOD OF WRITE-THROUGH OPERATION OF DIRECT VIEWING BISTABLE STORAGE TUBE TO PRODUCE NQNSTORED IMAGE OF HIGH BRIGHTNESS DURING SHORTA GE OF ANOTHER IMAGE Filed Oct. 16, 1967 Sheet 4 or 2 FIG. 1

HOR. VERT. POSlTlON 36 POSITION WAVE/"44 GEN.

FIG. 2

2 FIG. 3A [68 ON OFF 62 5 68 Q GRID 1 VOLTAGE J A L 60 CHESTER N. WINNINGSTAD INVENTOR BUCKHORN, BLORE, KLARQUIST & SPARKMAN ATTORNEYS 5, 1969 E c. N. WINNINGSTAD 3,43

METHOD OF WRITE-THROUGH OPERATION OF DIRECT VIEWING BISTABLE STORAGE TUBE TO PRODUCE NONSTOHED IMAGE OF HIGH BRIGHTNESS DURING SHOR'I'AGE OF ANQTHER IMAGE Filed Oct. 16, 1967 Sheet 2 of 2 FIG. 4 H

WRITING BEAM L CHARACTERISTIC 1 T I I I l BEAM I I I CURRENT 1 T l l 1 r I I. I l I l 1 ,.AV: P... ,,iAv P.

1 l 1 l l -v -|oo -ao 20 o +v can) TO CATHODE VOLTAGE TARGET FIG. 5

POTENTIAL 76 w DISTANCE Es W278 a -.A;=:" V qfl xu ,1" '1 IE. ME l A *1 Tl E TIME BOMBARDMENT A CURRENT I I AVERAGE 7. AVERAGE TIME CHESTER N. WINNINGSTAD INVENTOR BUCKHORN, BLORE, KLARQUIST 8. SPARKMAN ATTORNEYS United States Patent METHOD OF WRITE-THROUGH OPERATION OF DIRECT VIEWING BISTABLE STORAGE TUBE TO PRODUCE NONSTORED llVIAGE OF HIGH BRIGHTNESS DURING SHORTAGE OF AN- OTHER IMAGE Chester N. Winningstad, Portland, Oreg., assignor to Tektronix, Inc., Beaverton, Oreg., a corporation of Oregon Filed Oct. 16, 1967, Ser. No. 675,583 US. Cl. 315- 10 Claims Int. Cl. H01j 29/41 ABSTRACT OF THE DISCLOSURE A method of operating a direct viewing bistable storage tube is described involving write-through, which is the display of a nonstored charge image during bistable storage of another charge image on the same storage dielectric. The write-through light image of the nonstored charge image is provided with high brightness by interrupting bombardment by the writing beam of the storage dielectric portions corresponding to such nonstored charge image during writing. Interruption of the writing beam to reduce the bombardment time per unit area is achieved either by pulsing such beam on and off or by causing a dithering movement of the beam in a closed path on the storage dielectric which enables the current of the writing beam to be of a high value for greater stability.

Background of invention The subject matter of the present invention relates generally to a method of operation of a direct viewing bistable storage tube called write-through by which a nonstored charge image is displayed during the storage of another charge image on the same storage dielectric of the storage target of such tube. More specifically, the present method of write-through operation includes the step of interrupting bombardment by the writing beam of the storage dielectric portions corresponding to the nonstored charge image during writing to reduce the bombardment time per unit area while maintaining a high beam current. This beam interruption is accomplished either by pulsing such beam on and off, or by a dither movement of the beam in a closed path, to enable use of a higher peak beam current than would ordinarily be possible. As a result the light image display of the nonstored charge image is provided with a higher brightness substantially equal to that of the stored image.

Previously write-through operation was accomplished to a limited extent without decreasing the bombardment time per unit area, merely by decreasing the current of the writing beam to a low value below that sutficient to cause the potential of the charge image to exceed the critical minimum voltage necessary for bistable storage. However, there are several disadvantages to this prior art write-through method, one of which is that the light image display corresponding to the monstored charge image is of extremely low brightness, which is much lower than that of the stored image display. In addition, it is extremely difficult to adjust the writing beam current so that the write-through charge image will not be partially stored due to large changes in such current caused by small fluctuations in the power supply voltage providing 3,430,093 Patented Feb. 25, 1969 the grid-to-cathode bias voltage of the writing gun. This instability in the writing beam current increases as the cut-01f bias voltage is approached and is therefore more pronounced with a beam of low current like that used with the previous write-through method. Another more complicated write-through method is shown in US. Patent 3,277,333 of R. G. Williams in which the velocity of the writing beam is decreased to enable such writing beam to pass through a mesh storage target and strike a separate phosphor viewing screen without striking such target.

The method of the present invention overcomes the above discussed disadvantages by operating the writing beam at a high current during write-through and interrupting such current, by pulsing or dithering the beam as mentioned above, to reduce the bombardment time per unit area of the bombarded portions of the storage dielectric below that sufllcient to charge such portions to a potential greater than the critical minimum voltage necessary for bsitable storage. Greater writing beam stability results from using a high beam current so the small changes in grid bias voltage due to power supply ripple cause relatively small changes in beam current because the writing gun is biased on a more gradually sloping portion of its emission characteristic curve which is a plot of the log of the writing beam current versus grid-tocathode bias voltage. Because of this greater stability a higher average beam current may be employed with the present method without the potential of a portion of the Write-through charge image exceeding the voltage above which storage occurs, due to such power supply ripple. As a result, the light image display of the nonstored charge image produced during write-through is increased in brightness to a value substantially equal to that of the stored image display.

The low velocity flood electrons which uniformly bombard the storage dielectric during storage cause bistable storage of a charge image having a potential greater than the critical voltage, by increasing such potential to a first stable voltage corresponding to that of the collector electrode. However, such flood electrons function in an opposite manner with regard to the write-through charge image since its potential is less than the critical voltage, so that they tend to prevent storage by discharging such write-through charge image to reduce its potential down to a second stable voltage near the voltage of the flood gun cathode. This discharge action by the flood electrons operates as long as the potential of the write-through charge image lies below the critical minimum voltage necessary for bistable storage, which is sometimes referred to as the first crossover voltage on the secondary emission characteristic curve of the storage dielectric. For this reason it may be desirable to cut oif the flood electrons during the formation of the stored charge image but not during the formation of the nonstored writethrough charge image. Bistable storage is described in greater detail by US. Patent 3,293,473 of R. H. Anderson, which shows a simplified direct viewing bistable storage tube that may be operated by the method of the present invention.

By choosing the time of interruption of the writing beam which is proper with respect to the limit emission persistence of the phosphor material employed to produce the write-through light image of the nonstored charge image, the brightness of the write-through image can be maintained at a uniformly high level.

It is therefore one object of the present invention to provide an improved method of operation of a direct viewing bistable storage tube in which a light image corresponding to a nonstored charge image is produced with a high brightness during bistable storage of another charge image on the same storage dielectric of such tube.

Another object of the present invention is to provide an improved method of write-through operation of a direct viewing bistable storage tube in which a more stable writing beam of high current is employed by reducing the bombardment time per unit area during formation of the nonstored charge image, to provide a higher average beam current and a nonstored image display of high brightness which is substantially the same as that of the stored image display.

An additional object of the present invention is to provide an improved method of write-through operation of a direct viewing bistable storage tube in which a nonstored charge image is formed during storage of another charge image, by interrupting bombardment by a high current writing beam of corresponding storage dielectric portions, either by pulsing such beam on and off, or by dither movement of such beam in a closed path on the storage dielectric to reduce the bombardment time per unit area so that the instantaneous current of the flood beam may be greater than that which would ordinarily cause storage.

Brief description of the drawings Other objects and advantages of the present invention will be apparent from the following detailed description of certain preferred embodiments thereof and from the attached drawings of which:

FIG. 1 is a schematic diagram of a storage apparatus which may be operated in accordance with the method of the present invention;

FIG. 2 is an elevation view taken along the line 2-2 of FIG. 1 showing the light images of the stored and nonstored charge images formed by the storage tube;

FIG. 3A is a diagram of the writing gun grid voltage and the nonstored charge image voltage produced during the pulsed write-through method of one embodiment of the present invention;

FIG. 3B is an enlarged view of a nonstored display trace in FIG. 2 formed by the dither movement writethrough method of another embodiment of the present invention;

FIG. 4 shows the writing gun current emission characteristic;

FIG. 5 shows the potentials of charge images produced on the storage dielectric by different writing beams of high and low current, compared with the critical minimum potential above which storage occurs; and

FIG. 6 shows the average bombardment currents for different writing beams produced with high and low bias, compared with the critical minimum current necessary for bistable storage under the same conditions.

Detailed description of preferred embodiments As shown in FIG. 1, the write-through method of the present invention is used to operate a direct viewing bistable storage tube 10 which may be of the type described in US Patent 3,293,473 of R. H. Anderson or US. Patent 3,293,474 of C. B. Gibson. The storage tube has a storage target that includes a light-transparent support plate 12 of glass which may be the face plate of the tube envelope, a storage dielectric layer 14 of phosphor material coated on the support plate over a light-transparent conductive film 16 of tin oxide. The conductive film 16 serves as the target electrode and as a collector of the secondary electrons emitted by the storage dielectric providing such dielectric is in the form of separate dots or an integral layer sufliciently porous to enable such secondary electrons to be transmitted therethrough.

The storage tube also includes a writing gun comprising a cathode 18, a control grid 20 and a focusing and accelerating anode structure 22 for forming a narrow writing beam of high velocity electrons which are caused to strike the storage dielectric 14. The writing beam is deflected by signals applied to a pair of horizontal deflection plates 24 and a pair of vertical deflection plates 26 provided within the storage tube or corresponding magnetic deflection coils positioned outside of the tube, in order to form charge images on the storage dielectric 14. A pair of flood guns 28 and 30 are provided within the storage tube to uniformly bombard the storage dielectric 14 with low velocity flood electrons to cause bistable storage in a conventional manner of any charge images formed thereon whose potentials are greater than the critical minimum voltage necessary for bistable storage. The cathodes of the flood guns 28 and 30 are normally connected to ground through a switch 32 in the on position of such switch, while the collector electrode 16 is connected to a more positive D.C. supply voltage of about +200 volts, and the writing gun cathode 18 is connected to a highly negative DC. potential of about 4000 volts.

Under some circumstances it may be desirable to turn oil the flood electrons and prevent them from bombarding the storage dielectric during the formation of a charge image which is desired to be stored bistably. This is accomplished by moving the switch 32 to the off position which applies a voltage of about +100 volts to the flood gun cathodes, thereby biasing such flood guns to cut off. However, during the formation of the write-through charge image the flood guns are always turned on.

The vertical deflection plates 26 are connected to the output of a vertical amplifier 34, which may be a summing amplifier having two inputs. One input of amplifier 34 is connected to the movable contact of a potentiometer 36 whose opposite end terminals are connected, respectively, to sources of positive and negative D.C. supply voltage. The other input of the vertical amplifier is connected to the movable contact of a three-position switch 38. In the Store position of switch 38, the vertical amplifier is connected to the output of a preamplifier 40, whose input is connected to an input terminal 42 to which the vertical input signal whose waveform is to be displayed is applied. In the second position of switch 38 labeled Pulsed W.T. (which stands for pulsed write-through) the vertical amplifier is also connected to the output of preamplifier 40. However in the third switch position labeled Dither W.T. (which stands for dither write-through) the vertical amplifier is connected to one output of a sine wave generator 44 which applies a sine wave signal thereto.

The horizontal deflection plates 24 are connected to the output of a horizontal amplifier 46 which may also be a summing amplifier having two inputs. One input of amplifier 46 is connected to the movable contact of a potentiometer 48 whose opposite end terminals are connected, respectively, to sources of positive and negative D.C. supply voltage. The other input of the horizontal amplifier is connected to the movable contact of a three-position switch 50 which is ganged to the movable contact of switch 38. In the Store and the Pulsed W.T. positions of switch 50 the horizontal amplifier is connected to the output of a sweep generator 52 which produces a ramp shaped voltage as a horizontal sweep signal. The sweep generator may be triggered in response to the receipt of a vertical input signal to synchronize the sweep signal to such vertical input signal for non-periodic vertical input signals. This triggering of the sweep generator may be accomplished in the conventional manner by connecting such sweep generator to the output of the preamplifier 40. In the Dither W.T. position of the switch 50, the horizontal amplifier is connected to the other output of the sine wave gtnerator 44, which applies a horizontal sine wave signal thereto having the same amplitude but being out of phase with respect to the vertical sine wave millimeters. It should be noted that while the nonstored image displays 60 and 68 have been represented by dashed lines in FIG. 2 to distinguish them from the stored image display 58, such nonstored image displays 60 and 68 are actually continuous solid line light traces similar to the stored image display 58.

A curve 74 of the logarithm of the beam current variation caused by changes in grid-to-cathode bias voltage for the writing gun is shown in FIG. 4. The writing beam current characteristic 74 is more stable in a high current condition with a grid-to-cathode bias voltage near zero volts than in a low current condition with such a bias voltage near cut-off, which is about 100 volts. In other words, for a given change in bias voltage AV with the average bias set at volts, a corresponding change in writing beam current A1 is produced which is much less than the change in the current A1 produced by an equal variation in bias voltage AV at an average bias of 80 volts. This increase in instability with decreases of beam current occurs because the writing beam characteristic curve 74 has a much greater slope near cut-oif than near Zero bias. With the two embodiments of the write-through method of the present invention, the instantaneous writing beam current is at a high value corresponding to the bias voltage of -20 volts, while in the previous method of achieving write-through the writing beam current is set at a low value corresponding to the bias voltage of 80 volts. Therefore the Writing beam used in the present method produces a write-through charge image of more stable peak voltage.

As shown in FIG. 5, the critical minimum potential E for bistable storage varies with distance across the target in a manner similar to the curve 76 due to changes in the storage dielectric thickness, etc. It should be noted that the storage potential curve 76 only applies to a narrow horizontal portion of the target and slightly different curves would be produced for other target portions. Because of the variation in the critical storage potential E a guard band potential level 78 is selected slightly below the minimum value of the storage potential 76. This guard band potential E cannot be exceeded by the potential of the charge image formed on the storage dielectric in order to insure that no portion of such charge image is stored. In accordance with the write-through method of the present invention, a write-through charge image may be formed with an average maximum potential E only slightly below the guard band potential E This writethrough charge image potential 80 varies with power supply ripple. However, these potential variations are small due to the fact that the writing gun is biased in a high current condition, corresponding to the small changes of beam current Al for a given power supply ripple. However with low beam current condition a write-through charge image potential 82 is produced by the prior method which has a very large voltage variation corresponding to the large changes of beam current AI of FIG. 4, with the same power supply ripple. This means that the average maximum potential E of the write-through charge image produced by the low current method of the prior art cannot be made as close to the guard band potential E as potential E without causing bistable storage.

As shown in FIG. 6, the critical storage current I corresponding to the minimum voltage E necessary for bistable current is represented by line 84, While the guard band current level I is represented by the dashed line 86 slightly below the storage current I It should be noted that the storage current 84 of course changes with the bombardment time per unit area of storage dielectric and with variations in the minimum storage potential V But for a given bombardment time and storage dielectric area such minimum storage current can be accurately represented by line 84. The average bombardment current 88 of the present method which produces the charge image potential E of FIG. 5, is only slightly below the guard current 1,; because it has only a small variation caused by the power supply ripple. Similarly an average bombardment current which produces the charge image potential E of FIG. 5 is caused to have larger fluctuations by the same power supply ripple. It should be noted that the instantaneous writing beam current per unit area of the charge image actually varies from a maximum current much greater than the storage current level 84 to a zero current due to turning on and oif of the writing beam in the embodiment of FIG. 3A, or due to the dither movement of such writing beam in the embodiment of FIG. 3B. Thus the bombardment current 88 of FIG. 6 is the average writing beam current per unit area of charge image and not the actual writing beam current.

From the above it is clear that by applying the writethrough method of the present invention a stable nonstored charge image of greater potential can be produced by using a higher average bombardment current than in previous write-through methods. As a result of this higher average current, the light image display of the nonstored write-through charge image is of a greater brightness. In addition it is much easier to adjust the potential of the write-through charge image closed to the critical minimum voltage necessary for bistable storage without causing storage, due to the operation of the writing dot in a more stable condition. In addition, when the continuous annular trace 68 of the embodiment of FIG. 3B is produced with a sufliciently small diameter so that the opening in the center of such trace is not apparent to the eye, the visual effect is that of a brighter spot due to the larger unresolved area of such trace.

It will be obvious to those having ordinary skill in the art that many changes may be made in the details of the above-described preferred embodiment of the present invention without departing from the spirit of the invention. Therefore the scope of the present invention should only be determined by the following claims.

I claim:

1. A method of operating a direct viewing bistable storage tube to enable simultaneous display of a stored charge image and a nonstored charge image produced on the same storage dielectric of a storage target in said tube, comprising the steps of:

bombarding said storage dielectric with a first writing beam of high velocity electrons of sufficiently high current and bombardment time per unit area of storage dielectric to form a first charge image thereon of a potential greater than the critical minimum voltage necessary for bistable storage;

bombarding said storage dielectric with low velocity electrons to cause bistable storage of said first charge image and to produce a light image display corresponding to said first charge image;

bombarding said storage dielectric with a second writing beam to form a second charge image thereon during bombardment of said storage dielectric by said low velocity electrons; and

interrupting the bombardment of the storage dielectric portions corresponding to said second charge image by said second writing beam during the formation of said second charge image to reduce the bombardment time per unit storage dielectric area of said second writing beam to a value which causes the potential of said second charge image to remain below said critical voltage to prevent bistable storage of said charge image;

said second writing beam being of high current sufficient to produce a light image display corresponding to said second charge image of substantially the same brightness as the light image of the stored first charge image, and sufiicient to cause the potential of said second charge image to exceed said critical voltage but for said interrupting of said second writing beam.

2. A method in accordance with claim 1 in which the first and second writing beams are emitted from the same signal applied to switch 38. These sine waves cause a circular deflection of the writing beam to provide an annular trace on the storage tube as shown in FIG. 3B.

The control grid 20 of the writing gun is connected to the movable contact of still another three-position switch 54, which is ganged to the other two switches 38 and 50. In the Store and the Dither W.T. positions of switch 54 the control grid is connected to a source of negative D.C. supply voltage of -4020 volts to provide a grid-to-cathode reverse bias of 20 volts. This causes a writting beam of high current to be transmitted to the storage dielectric 14. In the Pulsed W.T. position of switch 54, the control grid 20 is connected to the output of a rectangular pulse generator 56 which applies rectangular voltage pulses of a maximum voltage of 4020 volts and a minimum voltage of -4l00 volts to the control grid which turns on the writing gun at such maximum voltage and turns ofl the writing gun at such minimum voltage. Thus when the writing gun is turned on the writing beam current is the same as when switch 54 is in the Store position, but the average current during pulsed write-through is less due to the fact that such writing beam is turned off for a portion of the time. Turning 01f the writing beam for a portion of the time allows the write-through charge image formed on the storage dielectric to be discharged by the flood electrons, thereby preventing bistable storage of such charge image.

When switches 38, 50 and 54 are in the Store positions, a first writing beam is caused to bombard the phosphor storage dielectric 14 to product a first charge image of sufficient potential to enable bistable storage by the flood electrons and such stored charged image is displayed as a light image 58 of high brightness as shown in FIG. 2. It should be noted that the storage tube may be of the mesh storage target type employing a separate phosphor viewing screen on which the light image 58 is produced by the flood electrons being transmitted through the mesh storage target on which the charge image .is formed.

Write-through of a nonstored charge image is accomplished with one embodiment of the present method by moving the switches 38, 50 and 54 to the Pulsed W.T. positions. This causes the rectangular pulse generator 56 to pulse a second writing beam on and off during formation of a second charge image on the storage dielectric 14 whose light image display 60 is shown in FIG. 2. As a result of this pulsed write-through operation the bombardment time per unit area of charge image is reduced to prevent the potential of the second charge image from exceeding the critical minimum voltage necessary for bistable storage. Thus the second charge image corresponding to waveform display 60, is not stored even though when the second writing beam is turned on it has a high current equal to that of the first writing beam which produced the stored charge image of waveform display 58. The nonstored waveform display 60 may be compared with the stored waveform image 58 to determine their relative rise times, amplitudes, pulse widths, etc. Also the shape of the nonstored waveform 60 may be changed to be similar to that of the stored waveform 58, and when this is achieved waveform 60 can then also be stored merely by switching to the Store position.

The pulsed write-through embodiment is shown in FIG. 3A to employ a pulsed grid voltage signal 62 which is applied to the control grid by the rectangular pulse generator 56 to turn the writing beam on and off. Also a charge image voltage 64 produced by the writing beam under such pulsed condition is shown related in time to the grid voltage signal. Thus, during the charging time T that the writing beam is turned on, the charge image voltage 64 increases from a rest potential E to a peak positive potential slightly less than the critical minimum voltage E necessary for bistable storage. When the writing beam is turned off the charge image voltage 64 is decreased from its peak potential to its rest potential E during the discharge time T This discharge time is controlled by the current of the flood guns 28 and 30 which cause such discharge. If the discharge time T is short compared to the light emission persistence of the phosphor employed as the storage dielectric 14, the light image of a stationary beam spot will remain at a uniform high brightness level at all times. For example, when Pl phosphor material having a persistence of 20 milliseconds is employed as storage dielectric 14, the discharge time T should be considerably less than 20 milliseconds because the persistence of a phosphor is the time it takes the light brightness to decrease to 10 percent of its maximum value. Therefore with a stationary writing beam the off time T should be on the order of about 1 millisecond or less for P-l phosphor.

Another embodiment of the method of the present invention is shown in FIG. 3B which involves a dither movement of the electron beam about a closed circular path 66 to produce an annular trace 68 on the phosphor storage dielectric 14. The writing beam strikes the storage dielectric 14 in a circular spot 70 of a diameter which may be about one-half to one-third of the diameter of the circular path 66 which such spot takes to form the annular trace 68. Thus it can be seen that the charge image of the annular trace 68 is discharging in all portions except that portion on which the writing beam spot 70 falls. Therefore the discharge time of such annular trace corresponding to T of FIG. 3A is the time it takes the beam spot to travel once about path 66. If the speed of the beam spot 70 and the length of the circular path 66 are of a proper value, the potential of the annular trace 68 will not exceed the critical minimum voltage necessary for bistable storage even though the current of the writing beam in the Dither W.T. position is the same as in the Store position of switches 38, 50 and 54.

It may also be desirable to combine the two writethrough methods of FIGS. 3A and 3B. Thus the writing beam may be pulsed on and off during the dither movement of the writing beam to produce the annular trace 68 in order to prevent such annular trace from storing when such writing beam is provided with a very high current. This may be accomplished by means of the rectangular pulse generator 56 by connecting the output of such pulse generator also to the Dither W.T. position of switch 54. It may be necessary to adjust the duty cycle of the rectangular pulse signal 62 to provide different charge times and discharge times, which may be accomplished by adjustment of a variable resistor 72 in the pulse generator 56 which serves as a duty cycle control.

As shown in FIG. .2, the annular trace 68 may be used as a pointer by positioning it adjacent the top of the trailing edge of the first cycle of stored waveform 58 and subsequently moving it to a position 68 adjacent the bottom of the leading edge of the second cycle of such waveform by adjustment of the movable contacts of the horizontal position potentiometer 48 and the vertical position potentiometer 36. In a similar manner the nonstored write-through trace 60 can be superimposed over the stored trace 58 by adjusting the vertical position control 36 to move such nonstored trace upward for more exact comparison of rise times, pulse widths, and signal amplitudes, etc. In addition to its use as a pointer the annular trace 68 may also be used to form letters or other characters on the storage dielectric by movement of the annular trace in a path having the shape of the character desired. In this case manual adjustment of the positioning voltages may be too slow, so that a character signal generator (not shown) is employed in place of the potentiometers 36 and 48. One example of an annular trace 68 suitable for the above mentioned uses may be produced by sine waves having a frequency of 1000 cycles per second with an ampltiude such as to cause the circular path 66 to have a diameter of about 25 millimeters when the beam spot 70 has a diameter of 8.25

source of electrons and are of substantially the same current.

3. A method in accordance with claim 1 in which the interrupting of the second writing beam is accomplished by pulsing said second writing beam on and oiT during the formation of said second charge image.

4. A method in accordance with claim 3 in which the second writing beam is deflected across the storage dielectric by a vertical input signal and a horizontal sweep signal during said pulsing.

5. A method in accordance with claim 1 in which the interrupting of the second writing beam is accomplished by moving said second writing beam around a closed path on the storage dielectric.

6. A method in accordance with claim 5 in which the second writing beam is deflected by a vertical sine wave and a horizontal sine wave of the same frequency and 90 degrees out of phase to move said second writing beam in a circular path to produce an annular second charge image of a diameter greater than the diameter of said second writing beam.

7. A method in accordance with claim 6 in which the annular, nonstored second charge image is moved with respect to the stored first charge image.

8. A method in accordance with claim 5 in which the second writing beam is also pulsed on and olf during movement about the closed path.

9. A method in accordance with claim 1 in which the low velocity electrons are prevented from bombarding the storage dielectric during the formation of the first charge image.

10. A method in accordance with claim 1 in which the storage dielectric is phosphor material which emits the light images of the first and second charge images.

References Cited UNITED STATES PATENTS 3,325,673 6/1967 Anderson 315-12 RODNEY D. BENNETT, Primary Examiner.

JEFFREY P. MORRIS, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 ,430 ,093 February 25 1969 Chester N. Winningstad It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below: In the heading to the drawing, sheets 1 and 2 line 3 thereof and in the heading to the printed specification, line 5 "SHORTAGE", each occurrence should read STORAGE Column 1 line 31 "invention" should read Invention Column 2 line 20, "bsitable" should read bistable line 63 "limit" should read light Column 4, line 73 "gtnerator" should read generator Column 5 line 10 "writting" should read writing line 30, "product" should read produce Column 7 line 63 "current" should read storage Column 8 line 22 "closed" should read closer Signed and sealed this 24th day of March 1970 (SEAL) Attest:

WILLIAM E. SCHUYLER, JR.

Commissioner of Patents Edward M. Fletcher, Jr. Attesting Officer 

